By John Davison and Jeff Mason
Syria's opposition has indicated it's ready for a two-week truce in Syria, saying it is a chance to test the seriousness of the other side's commitment to a US-Russian plan for a cessation of hostilities.
Combatants are required to say whether they will agree to the "cessation of hostilities" in the five-year war by noon local time on Friday, and to halt fighting on Saturday.
A statement seen by Reuters from the Saudi-backed High Negotiations Committee, which groups political and armed opponents of Syrian President Bashar al-Assad, said it "views a temporary two-week truce as a chance to establish how serious the other side is in committing to the points of the agreement".
But it objected to Russia being a guarantor of the truce alongside the US, saying Russia was a direct party to the conflict, and that the plan ignored the role Assad's allies Russia and Iran were playing.
In Washington, US President Barack Obama expressed caution about the plan to stop the fighting in Syria, which has killed 250,000 people and created a refugee crisis in Europe.
The last round of peace talks in Geneva broke up earlier this month without progress after the Syrian government launched a Russian-backed offensive on the city of Aleppo, where more fighting was reported on Wednesday.
Obama told reporters that if some progress was made in Syria, that would lead to a political process to end the war there.
"We are very cautious about raising expectations on this," he said.
Although US officials have raised the question of a political transition in Damascus, Assad, backed by Russia, shows no sign of stepping aside.
The cessation of hostilities plan does not include Islamic State or the Nusra Front, an al-Qaeda affiliate that is widely deployed in opposition-held areas.
The opposition has expressed fears government forces backed by the Russian air force will continue to attack rebels under the pretext of targeting the Nusra Front.
The Syrian government, its war effort buoyed since September by the Russian air force, has accepted the cessation of hostilities agreement announced on Monday.
Assad told Russian President Vladimir Putin on Wednesday his government was ready to help implement the deal.
Putin and Assad, who held a telephone conversation, stressed the importance of a continued "uncompromising" fight against IS, the Nusra Front and other militant groups.
US Secretary of State John Kerry said he had spoken to Russian Foreign Minister Sergei Lavrov and their teams would meet in the next day or so to discuss the planned ceasefire.
Putin has embarked on a round of telephone diplomacy, speaking to Assad, the Saudi king, the Iranian president and the Israeli prime minister. The Kremlin described the calls as an effort to explain the substance of the US-Russia-brokered ceasefire.
The Russian Defence Ministry said it had significantly reduced the intensity of its air strikes in Syria in the past two days in areas where armed groups had expressed their readiness to join the ceasefire.
Turkish President Tayyip Erdogan said he feared the ceasefire plan would do little more than benefit Assad.
Turkey has grown increasingly frustrated by the international response to the Syrian war, in particular US support for a Kurdish militia it sees as a hostile insurgent force.
The Syrian Kurdish YPG militia told Reuters on Wednesday it would abide by the plan to halt the fighting but reserved the right to respond if attacked.